In general, a coil spring is made of a coil wire whose cross section is circular. When loads act on this type of coil spring in the axial direction of coil, surface stresses that occur on the cross-sectional periphery in the coil wire become greater on the outer peripheral side of coil than those on the inner peripheral side thereof. This is because the surface stresses, which occur on the cross-sectional periphery of this coil spring, arise when the coil wire is affected by shear forces (or forces in shearing) in addition to the influences that the coil wire is curved (namely, torsional forces). Consequently, in this type of coil spring, cracks that become the cause of breakages are likely to occur on the inner peripheral side of coil where the surface stresses become higher.
Hence, in order that the surface stresses, which occur on the cross-sectional periphery of the coil wire when loads act on the coil wire in the axial direction of coil, are biased as less as possible, a coil spring with irregular cross section has been known, coil spring which is made of a coil wire whose cross section is formed as an irregular cross section that approximates an egg shape (see Japanese Unexamined Patent Publication (KOKAI) Gazette No. 59-190,528, for instance).
As illustrated in FIG. 8, this coil spring with irregular cross section has an egg-shaped portion 81 on the inner peripheral side of coil in the vertical cross section that is taken along the axial direction of coil in a coil wire 80; and has a flattened portion 82 on the outer peripheral side of coil therein. That is, the vertical cross-sectional profile of, the coil wire 80 is constituted of the curve-lined section (i.e., C-B-A-E-D) and the liner-lined section (i.e., C-D). Note that, of the vertical cross-sectional profile of the coil wire 80, the part that is the closest to the central axis of coil is adapted into an innermost side end “A” of the coil spring with irregular cross section. Moreover, in the present specification, the term, “inner peripheral side of coil,” means that it is the inner peripheral side of coil (or the central side of coil spring) in the vertical cross section that is taken along the axial direction of coil in coil wire, unless otherwise specified; and the term, “outer peripheral side of coil,” means that it is the outer peripheral side of coil (or the outwardly opposite side to the central side of coil spring) in the vertical cross section that is taken along the axial direction of coil in coil wire, unless otherwise specified. The gazette that describes the above coil spring sets forth that disposing the flattened portion 82 at the outermost side section in the coil spring makes it possible to make stress sharing greater on the outer peripheral side of coil when loads act thereon in the axial direction; as a result, it is possible to equalize surface stresses on the cross-sectional periphery in the coil wire 80 when loads act thereon in the axial direction.